Enhanced thermohydraulic performance in helical coil heat exchangers using synthesized Cu-doped TiO2 nanofluids: An experimental approach

With increasing global energy demand, environmental concerns, and the depletion of traditional resources, enhancing the efficiency of energy-intensive systems such as heat exchangers has become imperative. This study addresses this challenge by synthesizing copper-doped TiO₂ nanoparticles via a sol–gel method, aiming to simultaneously improve heat transfer rates and reduce pumping power—thereby overcoming the common issue of increased flow resistance in nanofluid applications and enhancing thermohydraulic performance in helical coil heat exchangers. Three Cu-doped TiO₂ samples were prepared with doping levels of 1 %, 3 %, and 5 % Cu, and characterized using FTIR, XRD, DLS, ICP-OES, and TEM analyses, confirming successful Cu²⁺ incorporation and improved dispersion stability. Nanofluids based on pristine TiO₂ and the Cu-doped TiO₂ (5 wt% in deionized water) were tested in a horizontally oriented helical copper coil (inner diameter: 9.5 mm) across a Reynolds number range of 5,000 to 17,000. Cu doping markedly improved convective heat transfer, with TiO₂–Cu₅ achieving 144.8 % maximum enhancement and a Synergistic Heat Transfer Enhancement of 56.5 %. While friction rose at low flow rates, drag reduction up to 24.6 % occurred at higher Reynolds numbers. Exergy efficiency also increased, peaking at 58 % for TiO₂–Cu₅, which reached the highest thermal–hydraulic enhancement factor of 2.25. These results demonstrate, for the first time, that Cu-doped TiO₂ nanofluids can simultaneously enhance heat transfer and flow efficiency in heat exchangers. This study presents the first correlations for Nusselt number and friction factor of TiO₂ and Cu-doped TiO₂ nanofluids in a helical tube, covering laminar and turbulent regimes.